1. Field of the Invention
The invention relates to a process for polymerizing 1,3-diisopropenylbenzene, to polyindanes resulting from the process, to compositions for preparing low dielectric constant coatings, comprising polyindanes in admixture with 1,3-diisopropenylbenzene and to processes for coating substrate with the low dielectric coatings.
2. Information Disclosure
Low dielectric constant insulators are critical components in on-chip interconnect and first lever package integrated circuit applications. Such materials fulfill several functions. First, they provide electrical and electronic isolation of one discrete chip component from another and permit high component densities by eliminating crosstalk between the discrete components. Second, they function to planarize the topography of a chip during many fabrication steps. Lastly, they act as barriers to contaminants and corrosive agents and to low energy .alpha.-particle penetration. Because of their importance, much research has been directed towards the development of novel low dielectric constant insulators in general and to organic dielectric materials in particular. The focus on organic dielectrics has derived mainly from the ability of these materials to be easily prepared and applied using solution or vapor deposition techniques. Besides having a low dielectric constant, an organic dielectric must also possess good thermal stability, a low coefficient of thermal expansion, good adhesion to various substrates, low water absorption and a high glass transition temperature.
Few organic dielectric materials have all of the desired properties. For example, polyimides show excellent processibility, thermal and adhesion properties, but have dielectric constants in the range of 3.5-3.6 (as compared to polystyrene which has a dielectric constant of 2.55). Furthermore, polyimides absorb water, a phenomenon which gives rise to even higher dielectric constants when polyimides are stored under conditions of high relative humidity. As the dielectric constant increases, the density at which elements can be packed on a chip decreases. For the past two decades, the trend has been toward increasing density of discrete devices as the size of the devices shrinks. Consequently, polymeric alternatives to polyimides with lower dielectric constants are being sought.
Colvin and Muse (Chemtech 1986, 500-504) have disclosed the cationic polymerization of 1,4-diisopropenylbenzene using a proprietary catalyst. The products of the polymerization using the undisclosed catalyst were stated to have a high glass transition temperature (T.sub.g) and good thermal stability. However, 1,4-diisopropenylbenzene is a solid and cannot be directly spun onto a silicon wafer or photopolymerized without the use of a solvent.
D'Onofrio [J Appl. Polym. Sci. 8, 521 (1964)] has also reported the cationic polymerization of 1,4-diisopropenylbenzene, but Colvin and Muse indicate that the resulting polymer had a low softening point and contained a large amount of HCl which was slowly released on standing.
In contrast to 1,4-diisopropenylbenzene, 1,3-diisopropenylbenzene is a free-flowing, colorless, mobile liquid with a high boiling point (231.degree. C.). If it could be efficiently polymerized and if the product were a low dielectric, thermally and dimensionally stable polymer, a most useful process would be provided.
Lutz et al. [Makromol. Chem. 183, 2787-2797 (1982)] reported that when 1,3-diisopropenylbenzene was polymerized with an anionic organometallic initiator, the reaction led initially to a polymer that appears to be predominantly a polypropylene with pendant isopropenyl phenyl residues: ##STR1## The polymer depolymerizes at slightly above room temperature to revert to the monomer. When allowed to stand at -30.degree. C. the linear polypropylene begins to crosslink, but no polymer that is stable at room temperature is described. p The use of 1,3-diisopropenylbenzene as a comonomer in cationic polymerizations is known ("see European published applications 449,453 and 466,305). There is an early reference to the cationic polymerization of "diisopropenylbenzenes" and an implication that 1,3-diisopropenylbenzene forms polymers that contain indane structures. [Brunner et al. J Polym. Sci 28, 629-631 (1958)]. The polymers are said to be soluble in a wide range of solvents. U.S. Pat. No. 5,079,378 (Crivello) suggests a long list of possible monomers that might be polymerized by the process of the patent, but, although 1,3-diisopropenylbenzene is listed, there is no indication that such a reaction was actually tried and no characterization of either the product of such a reaction or of its particularly and unexpectedly useful properties.